In addition to gaining energy from the ingestion of nutrients from foods and beverages (exogenous


sources), the human body has the ability to store and utilize substrates for later use (endogenous sources). Under normal circumstances, the primary nutrients for energy production come from carbohydrate and fat. Usually, protein contributes less than 5% to 10% to the body’s energy needs because it has so many other important functions, such as acting as enzymes, hormones, and immune proteins and performing func- tions such as tissue maintenance, growth, and repair. Te exception to this is during the later stages of prolonged endurance exercise where proteins can contribute up to 15% of the energy needs if exogenous carbohydrate is not consumed and glycogen stores become depleted. In this chapter, we will focus on car- bohydrate and fat being the fuel sources for working muscle during exercise. Te storage form of carbohydrate in humans, glycogen, is found in both skeletal muscle and liver.


Glycogen storage has a maximum capacity, and, on average, these organs can store approximately 1,600 to 2,500 kcal of energy. Te liver can store up to 75 to 100 g (300 to 400 kcal) of glycogen, while skeletal muscle can hold up to 300 to 400 g (1,200 to 1,600 kcal). Plasma glucose is a small source of carbo- hydrate, containing approximately 5 to 25 g (20 to 100 kcal). Under normal circumstances, we cannot fully depend on any of these stores to fuel exercise because the brain and central nervous system require glucose and take priority. Fat (adipose tissue) is stored as triglycerides within adipose tissue and skel- etal muscle. Unlike glycogen, adipose tissue triglyceride is limitless and can provide 70,000 kcal of energy or more.3,34,35


Energy Systems


ATP is stored in body cells in very limited quantities, so the body must constantly have the ability to gen- erate ATP to meet the demands of cellular metabolism and especially muscle contraction during exercise. During rest, most of the cells and organs use a constant supply of ATP. With increased activity, skeletal muscle ATP demand can increase greatly, depending on the intensity and duration of exercise. As ATP is used—with each muscle contraction generating ADP and a single phosphate group—the ADP and phos- phate group must be rephosphorylized to ATP. Tere are three basic metabolic pathways to accomplish this: those that generate ATP without the use of oxygen, called anaerobic; those that require oxygen involve- ment, called aerobic; and those that rely on the large amount of creatine found in muscle cells, called the phosphocreatine (PC) system. Approximately 95% of the body’s creatine is found in skeletal muscle and can easily be replenished by the diet. Athletes who include meat in their diet obtain plenty of creatine, but creatine can also be synthesized from the amino acids methionine, arginine, and glycine, which are obtained from other dietary sources.1,3,34,35


ADENOSINE TRIPHOSPHATE–PHOSPHOCREATINE SYSTEM


Te most rapid method that the body uses to produce ATP is the ATP-PC system. Tis anaerobic system is used during intensive, explosive movements, such as a tennis serve or a power liſt. Tis system generates ATP rapidly; however, it is very limited and only supplies ATP for up to 10 seconds. A key component in the ATP-PC system is PC, which is stored in skeletal muscle. PC is simply a molecule of creatine attached to a single molecule of phosphate. A creatine kinase enzyme breaks the two molecules apart, and the free phosphate molecule then combines with ADP, forming new ATP. Te ATP-PC system is limited because there is a very small pool of PC stored in the skeletal muscle. Once all PC molecules have donated their phosphate group to ADP, the system can no longer facilitate further ATP production until PC is replen- ished, which normally happens during recovery with rest and dietary intake.1,3


ANAEROBIC GLYCOLYSIS


Tis system oxidizes one glucose molecule (six carbons) to form two pyruvic acid molecules (three car- bons). Glucose can come from dietary intake (circulating blood glucose) or from glycogen stored in the muscle or liver. In the process, potential energy is generated in two ways. First, ATP is directly generated from the breakdown process of glucose to pyruvate. Second, while glucose is being oxidized, hydrogen


06 SECTION 1: SPORTS NUTRITION BASICS


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